This invention relates generally to the field of geophysical prospecting. More particularly, the invention is a method of suppressing multiples in the collection of marine seismic data.
In marine seismic data, one of the most significant sources of noise is reverberation caused by up-coming seismic waves that have previously reflected off the air-water interface. This type of noise is called xe2x80x98multiplesxe2x80x99 and it re-illuminates water-bottom and sub-surface reflectors with time latency. Consequently, multiples generate false images of the sub-surface and hamper accurate interpretation of marine seismic data. The purpose of the present invention is to suppress or reduce multiple energy at the time of data collection.
There exist several data processing methods to eliminate or reduce the multiples in seismic data. Most of these methods are based on filtering multiples out of the data by taking advantage of their unique properties. These properties include predictive repetition with a constant amplitude decay rate and move-out difference from the primary signal. Other more advanced data processing methods involve modeling the multiples and solving the full wave equation.
Another class of methods to reduce multiples in intermediate depths of water uses a configuration of marine seismic data acquisition called xe2x80x98ocean bottom cablexe2x80x99 (OBC). This type of acquisition positions different sensors, typically hydrophones and geophones, in a cable at the water bottom to measure both pressure and particle velocity waves, respectively. Combined with a proper processing algorithm, these methods can subtract a portion of multiple energy from the collected seismic data.
Multiple contamination is a fundamental impediment to use of detailed analysis of seismic amplitude data in many prospect areas. Complex multiples severely corrupt target horizons rendering structural and basic stratigraphic mapping difficult and seismic amplitudes of little value. Aggressive application of current mitigation methods has failed to substantively improve data quality, especially when the multiples are more than an order of magnitude stronger than the primary signals of interest.
W. H. Clark""s U.S. Pat. No. 4,625,302, xe2x80x9cAcoustic Lens for Marine Seismic Data Multiple Reflection Noise Reductionxe2x80x9d, issued Nov. 25, 1986, discloses a method for multiple reduction in marine seismic data acquisition using air bubbles. Clark""s ""302 patent relies on a refraction mechanism achieved by lowering local sound speed relative to nominal water sound speed. This is accomplished by a bubble emitter configured to produce an acoustic lens of air bubbles. The air bubble lens is designed to refract, rather than reflect, the multiples laterally and thus away from the receivers.
P. D""Antonio et al. in U.S. Pat. No. 5,401,921 xe2x80x9cTwo-Dimensional Primitive Root Diffusorxe2x80x9d, issued Mar. 28, 1995 to RPG Diffusor Systems, Inc., discloses a two-dimensional acoustic diffuser based on a primitive root sequence. This diffuser is constructed with solid materials, not air-bubbles. It has a relatively small scale (2 ft by 2 ft for one period of two-dimensional primitive root sequence) because it is designed for diffusing sound wavelengths in air. In addition to the phase grating, it also relies on sound absorption by the back plane. The main application is for suppressing sound in confined air spaces such as music halls and recording studios.
W. H. Berhens"" U.S. Pat. No. 5,959,938, xe2x80x9cTuned Bubble Attenuator for Towed Seismic Arrayxe2x80x9d, issued Sep. 28, 1999, discloses a method to attenuate horizontally transmitted acoustic energy in a marine environment by emitted bubbles. Berhens"" ""938 patent uses the attenuation mode, rather than the reflection mode, by attempting to create relatively large ( greater than 9 mm) bubbles at the resonant frequency of the seismic wave. The emitted bubbles are localized near the seismic source and their purpose is to restrict side-directional propagation to enhance signal-to-noise ratio, rather than to attenuate multiples.
The present invention is an apparatus for suppressing multiples in the collection of marine seismic data. The apparatus comprises at least one source positioned in the body of water; at least one receiver positioned in the body of water below the air-water interface and near the sources; a bubble diffuser positioned in the body of water so that the bubbles emitted by the bubble diffuser are positioned between the receivers and the air-water interface, wherein the emitted bubbles provide high acoustic reflection and substantially suppress specular reflection of seismic waves; and a control for activating the bubble diffuser during the collection of seismic data by the sources and receivers.
The present invention is also a method for suppressing multiples in the collection of marine seismic data. At least one source and at least one receiver are positioned in a body of water, with the receivers positioned below the air-water interface. A bubble diffuser is positioned in the body of water so that the bubbles emitted by the bubble diffuser are positioned between the receivers and the air-water interface. The emitted bubbles provide high acoustic reflection and substantially suppress specular reflection of seismic waves. The bubble diffuser is activated during the collection of seismic data by the sources and receivers.